1. Field of the Invention
The present invention relates to an oriented polyester film. More particularly, it relates to an oriented polyester film applicable to a base film for a magnetic recording medium, which is excellent in slipperiness and endurable travelling performance.
1. Description of the Prior Art
In general, polyester films such as those of polyethylene terephthalate are excellent in physical and chemical characteristics and used as films for magnetic tapes, floppy disks, photographic films, films for condensers, packaging films, Roentgen films and microfilms in addition to films for fiber and molded substances. Slipperiness, abrasion resistance and the like of such polyester films exert serious influence on workability in film production steps and the quality of the produced films. Particularly when a polyester film is coated on its surface with a magnetic layer to be applied to a magnetic tape, friction and abrasion are remarkably caused between the film surface and a coating roller for applying the magnetic layer, resulting in wrinkles and scratches on the film surface. After the polyester film coated with the magnetic layer is slit to be worked into an audio tape, video tape or a computer tape, friction and abrasion are remarkably caused between the tape and a guide portion, a reproduction head or the like when the tape is taken up from a reel or cassette or wound around the same, leading to scratches and distortion of the tape. Further, white pulverulent material is deposited by scraping of the polyester film surface, to cause lack of magnetic recording signals, i.e., drop-out. Coarse projections caused by agglomeration of added particles imperfectly dispersed in the polyester film and large size of the particles also lead to a disadvantage such as the drop-out.
In order to improve slipperiness and endurable travelling performance of such a film, the film surface is generally irregularized to decrease the contact area between the film and a guide roll or the like. Such surface irregularity is generally formed by a method of depositing insoluble particles from the catalyst residue in a polymer employed as the material for the film as disclosed in U.S. Pat. No. 4,067,855 or a method of adding inactive inorganic or organic particles as disclosed in U.S. Pat. No. 3,980,611, for example. In general, slipperiness of the film is improved as the particles in the polymeric material are increased in size, whereas such increased particle size itself causes the disadvantage such as drop-out in precision use such as application to a magnetic tape and significant deterioration of electromagnetic transformation characteristics. Thus, the irregularity of the film surface must be precisely adjusted in configuration, density, height and the like.
The size distribution of the particles has been generally comprehended only in view of the average particle size while the particles are not controlled in size distribution and configuration, whereby it has been difficult to obtain a film surface configuration in design.